An example of information output apparatuses for a word processor, personal computer, facsimile apparatus, and the like is a printer for printing information such as desired characters or images on a sheet-like printing medium such as a paper sheet or film.
The printing method of the printer includes various known methods such as a thermal method and ink-jet method. In particular, the ink-jet method of discharging ink to print information has recently received a great deal of attention because of non-contact printing on a printing medium such as a paper sheet, and easy color printing.
The ink-jet arrangement comprises a printing head for discharging ink in accordance with desired print information. The printing head prints information while reciprocating in a direction perpendicular to the feed direction of a printing medium such as a paper sheet. In general, this serial printing method is widely adopted in terms of low cost and easy downsizing.
The ink-jet printing head has ink-jet discharge nozzles serving as a plurality of aligned printing elements, and is mounted on the carriage of the printer main body. While the printing head is moved by the carriage, it prints information by discharging ink.
Examples of the driving method are an all-nozzle discharge method of discharging ink from all the nozzles at once, and a time division discharge method of discharging ink by time division by grouping nozzles to be driven into several blocks.
The all-nozzle discharge method requires a large power supply in order to simultaneously drive all the nozzles. This method is not suitable for an ink-jet printer which aims at low cost and small size.
In the time division discharge method of discharging ink by grouping nozzles into several blocks and driving them by time division, large power need not be supplied at once. Thus, the time division discharge method is employed in an ink-jet printer which aims at low cost and small size.
For example, a printing head shown in FIG. 1A has 32 nozzles 1 to 32. The 32 nozzles discharge ink every four nozzles in one driving operation. The printing head is divisionally driven eight times to discharge ink from all the 32 nozzles (for one column).
More specifically, the 32 nozzles are grouped into eight blocks: the first block (1, 9, 17, and 25), the second block (2, 10, 18, and 26), the third block (3, 11, 19, and 27), . . . , the eighth block (8, 16, 24, and 32).
Nozzles belonging to the first block (1, 9, 17, and 25) are simultaneously driven (discharge ink) at a timing B0; nozzles belonging to the second block (2, 10, 18, and 26), at a timing B1; and nozzles belonging to the third block (3, 11, 19, and 27), at a timing B2. Finally, nozzles belonging to the eighth block (8, 16, 24, and 32) are simultaneously driven (discharge ink) at a timing B7 to complete discharge of one column (all the 32 nozzles).
FIG. 1B shows lines of print dots (discharge dot layout) formed using the printing head in FIG. 1A.
In the printing head shown in FIG. 1A, the printing head is divisionally driven eight times. A dot layout formed by discharge of one column (32 nozzles) is represented by ● in FIG. 1B. One vertical line shown in FIG. 1B is formed by discharge of four columns (32 nozzles×4).
For descriptive convenience, FIGS. 1A and 1B show an 8-block, 32-nozzle printing head. The printing head may be an 8-block, 64-nozzle printing head, or a printing head having about several hundred nozzles in eight or 16 blocks.
As shown in FIG. 1A, the carriage is inclined at an angle θ from the printing head. While the carriage moves a printing head 1 in the main scanning direction, the printing head discharges ink. Vertical lines as a discharge dot layout shown in FIG. 1B become straight line by line.
In this manner, the printing head shown in FIG. 1A is driven by the time division discharge method of grouping nozzles into several blocks, and driving the nozzles by time division, thereby discharging ink. This printing head can provide, e.g., a discharge dot layout of straight vertical lines, as shown in FIG. 1B, at low cost and small size without supplying large power at once.
In the example of FIGS. 1A and 1B, the 32 nozzles are grouped into eight blocks each including four nozzles. Ink is discharged in a 1-block (4-nozzle) unit. All the 32 nozzles (one column) can discharge ink by eight discharge operations.
FIG. 2 shows an arrangement of the printing head. In FIG. 2, reference numeral 100 denotes a printing head main body driving section. In this example, the driving section 100 has 64 ink-discharge nozzles.
The 64 nozzles are driven every eight nozzles shown in FIG. 3 as one block. All the 64 nozzles discharge ink by eight driving operations.
More specifically, all the 64 nozzles are grouped into block a (1, 9, 17, 25, 33, 41, 49, and 57), block b (4, 12, 20, 28, 36, 44, 52, and 60), block c (7, 15, 23, 31, 39, 47, 55, and 63), block d (2, 10, 18, 26, 34, 42, 50, and 58), block e (5, 13, 21, 29, 37, 45, 53, and 61), block f (8, 16, 24, 32, 40, 48, 56, and 64), block g (3, 11, 19, 27, 35, 43, 51, and 59), and block h (6, 14, 22, 30, 38, 46, 54, and 62).
In FIG. 2, ink is discharged from the respective nozzles under heating control of ink within the nozzles by using a heat enable signal 101, block enable signal 104, and latch enable signal 106.
The heat enable signal 101 is a signal for permitting heating of a nozzle. The block enable signal 104 is a signal for permitting heating of nozzles belonging to a block to be selected (to be driven). A latch enable signal 106 is a signal for permitting heating of a predetermined nozzle to be selected (to be driven).
If the heat enable signal 101 and block enable signal 104 are selected, and the latch enable signal 106 (in the presence of image data for causing a nozzle at a predetermined position to discharge ink) is selected, a predetermined nozzle is heated to print information on a printing medium by ink discharge.
More specifically, in FIG. 2, the block enable signal 104 (3 bits in this example), and a decoder 108 for generating a block selection signal 109 for selecting a block designated by the block enable signal 104 exist to drive the 64 nozzles grouped into eight blocks.
Image data is temporarily sent to an image data latch 103 together with a data clock signal 105 and the latch enable signal 106. After the image data latch 103 holds all signals necessary to drive all the nozzles, data 107 is transferred to a designated nozzle.
FIG. 4 is a timing chart showing respective printing driving control signals. The block selection signal 109 sequentially operates (enables) the blocks in order to go through the respective grouped blocks (8 nozzles×8 blocks) once. In the example of FIG. 4, the block selection signal 109 sequentially enables block 0 to block 7. In FIG. 4, the latch enable signal 106, serial image data signal 102, and data clock signal 105 transmit next data.
As described above, the nozzles in the printing head main body driving section 100 are driven by driver switching using an AND output of the three, block selection signal 109, heat enable signal 101, and intra-latch data 107.
In recent years, demands have arisen for ink-jet printers which operate at high speed. To meet these demands, printing heads having a larger number of nozzles are required.
To implement a low-cost, small-size ink-jet printer, the time division discharge method of grouping nozzles into several blocks and driving the nozzles by time division so as to eliminate the necessity for supply of large power at once must be adopted. Further, the number of nozzles must be increased along with the increase in speed.
However, the following two problems arise when nozzles are grouped into several blocks, the nozzles are driven by the time division discharge method, and the number of nozzles (nozzle density) present in the printing head is increased to cope with the increase in speed.
First, the image quality is degraded by pressure interference (crosstalk) generated in ink discharge.
The printing head receives interference (crosstalk) owing to the pressure between nozzles that is generated in ink discharge. The printing density changes every discharge nozzle in accordance with the nozzle driving order, resulting in low image quality. The influence of the pressure interference (crosstalk) becomes more prominent as the number of nozzles (nozzle density) present in the printing head increases. This degradation in image quality must be prevented even if the number of nozzles is increased to cope with the increase in speed.
This will be explained in more detail. In the ink-jet printer, ink vibrates within the printing head after the nozzle of the printing head discharges ink. The vibrations influence ink discharge in the next period. When ink expands externally from an orifice owing to the vibrations, an ink droplet to be discharged in the next period becomes larger than the normal ink droplet. When ink contracts internally from the orifice, an ink droplet to be discharged in the next period becomes smaller than the normal ink droplet. In this way, ink discharge in the next period is influenced by the vibrations, decreasing the image quality of a printed image. If ink is discharged after ink vibrations settle so as to prevent the influence of vibrations on ink discharge in the next period, the printing speed decreases. In the prior art, the nozzles of respective grouped blocks are driven in a fixed order, ink vibrations in the printing head may greatly vary periodically, and the influence of the vibrations becomes serious. It is, therefore, difficult to achieve both prevention of degradation in image quality and high-speed printing.
Second, if the number of nozzles is increased along with the increase in speed, the number of signal lines increases due to an increase in the number of nozzle control block enable signals.
In the above-described decoder system, an increase in the number of nozzles in the time division discharge method increases the number of nozzle blocks. The number of block enable signals for selecting blocks increases, and the number of signal lines also increases.
For example, when the number of nozzle blocks is 8, as shown in FIG. 2, the block enable signal for selecting blocks requires only 3 bits. If the number of nozzle blocks increases to, e.g., 16, the block enable signal must require 4 bits.
As the first method of decreasing the number of block enable signal lines, Japanese Patent Laid-Open No. 06-305148 discloses a method using a block clock and ring counter.
More specifically, as shown in FIG. 5, Japanese Patent Laid-Open No. 06-305148 discloses a method of mounting a ring counter 309 in a printing head, and generating block enable signals 301 to 308 for selecting the blocks of nozzles to be driven by a signal from the ring counter 309.
FIG. 6 shows the waveforms of signals used in this arrangement. In FIG. 6, reference numerals 401 to 403 denote image data transfer signals. The ring counter 309 is operated by a block clock signal 404 to obtain the ring counter outputs 301 to 308. As is apparent from the comparison between the waveforms of the block enable signals 301 to 308 in FIG. 6 and the block selection signal 109 output from the decoder 108 in FIG. 4, the waveforms of the block enable signals 301 to 308 in FIG. 6 play the same role as the block selection signal 109 output from the decoder 108 in FIG. 4.
As the second method of decreasing the number of block enable signal lines, there is proposed a method of transmitting a data signal and block enable signal by using the same signal line. For example, the data signal is sent in the same data unit as that of the block enable signal, and the block enable signal for the data is always sent before the data signal.
To divide a 64-bit data signal into eight blocks and transmit the divided data in the second method, (8 data bits+3 block bits)×8=88-bit data signal must be transferred. Compared to transfer of only a data signal, the data signal transfer amount becomes 1.375 times.